1. Field of the Invention
The present invention relates to a flux transformer. More specifically, the present invention relates to a new construction of a flux transformer formed of an oxide superconducting thin film and a method for manufacturing the same.
2. Description of the Related Art
A SQUID (Superconducting Quantum Interference Device) is a device employing special characteristics of superconducting materials. A SQUID is formed with a superconducting loop containing a weak junction and can be used as a magnetic flux sensor with high sensitivity. In many cases, a SQUID is used together with a flux transformer. When a SQUID is used with a flux transformer, the sensitivity of a magnetic flux sensor may be increased and the choice for arranging the physical layout are increased.
A flux transformer comprises a pair of coils and these coils are connected with a pair of lines. Generally, these coils and lines are formed of a superconducting material in a form of, for example, superconducting thin film.
In this kind of a flux transformer, one of the coils has several turns. Accordingly, it is unavoidable that one of the lines intersects the coil having several turns. The line that intersects the coil is called a bridge part. Furthermore, a non-superconducting thin film is inserted between the bridge part and the coil so that both of them are insulated from each other.
One of the methods for producing a flux transformer formed of an oxide superconducting thin film is described on pp. 123-125 of "Applied Physics Letters 59(1), 1 Jul. 1991". This method is described below.
At first, an oxide superconducting thin film is deposited on a substrate and patterned to have a pattern of a first superconducting thin film. Namely, the pattern of the first oxide superconducting thin film corresponds to the lines except the bridge part, the pickup coil and the input coil. Successively, a non-superconducting thin film is deposited on the surface of the first oxide superconducting thin film and the substrate and a part of the non-superconducting thin film is removed. As a result, one end of the input coil and one end of the pickup coil appear forward. Finally, a second oxide superconducting thin film is deposited. The second oxide superconducting thin film is patterned to be a bridge part.
Another method for producing a flux transformer having substantially the same construction is described on pp. 988-990 of "Applied Physics Letters 59(8), 19 Aug. 1991". In this method, only the bridge part formed of an oxide superconducting thin film is patterned on the substrate at first. Successively, a non-superconducting thin film is formed on the substrate and the first oxide superconducting thin film, and two parts of the non-superconducting thin film are removed at both ends of the first superconducting thin film. Finally, the second superconducting thin film is deposited and patterned to form a remaining pattern of the flux transformer on the non-superconducting thin film.
Still another method for manufacturing the magnetic flux transformer is described on pp. 2336-2338 of "Applied Physics Letters 56(23), 4 Jun. 1990" and on pp. 2871-2873 of "Applied Physics Letters 62(22), 31 May 1993". In this method, a superconducting thin film is patterned by using masks made of a metal or a silicon.
The methods described above have the following problems.
Concerning the first method, the quality of the first superconducting thin film that was deposited is apt to be low in the completed flux transformer. Because the first superconducting thin film is often damaged during the deposition of the non-superconducting thin film and the second superconducting thin film.
Concerning the second method, the quality of the second superconducting thin film that was deposited is apt to be low. The reason. is that the quality of the support layer influences the quality of the second oxide superconducting thin film. However, it is difficult to uniformly deposit a non-superconducting thin film having high quality on the entire surface of the substrate. Accordingly, it is difficult to deposit an oxide superconducting thin film having high quality on a non-superconducting thin film not having high quality.
Furthermore, concerning the third method using a metal mask or silicon mask for patterning, the problem is that processing precision at patterning is low. Using this method, the accuracy is at best only 20 .mu.m degree.